Optical scanning device

ABSTRACT

In a combined optical and magneto-optical recorder, data stored in pits and a magnetic layer may be simultaneously detected with an optical scanning device. Reflected light is divided by a polarizer and directed to one of two photodetectors depending on the polarization of the reflected light. A magneto-optic recorded data signal (containing cross-talk) is detected by differencing the signals provided by the two photodetectors. Reflected light is also directed through a λ/4 polarizer and a second beam divider to third and fourth photodetectors depending upon the polarization of the light passing the λ/4 polarizer. A second difference signal, which essentially conforms to the cross-talk, is generated from the differences of signals provided by the third and fourth photodetectors. The two difference signals are combined to produce a cross-talk free magneto-optic recorded data signal.

The present invention relates to an optical scanning device which issuitable for reading both an optical and a magneto-optic recordingmedium as well as a combined magneto-optic recording medium upon whichitems of data are stored on top of one another in both a magnetic layerand by means of so-called pits, wherein the light beam from a lightsource is focused on the recording medium and is reflected from therecording medium through a first λ/2 plate to a first polarization beamsplitter which guides the light beam onto a first or secondphotodetector, wherein the output of the first and of the secondphotodetector are connected to the inputs of a first differentialamplifier.

BACKGROUND OF THE INVENTION

A known optical recording medium is, for example, the CD disc in which alight reflecting aluminum layer follows on the transparent layer. Thelight reflecting aluminum layer has depressions, so-called pits, whichrepresent the items of data stored on the CD disc. The items of data arereadable from the CD disc by means of an optical scanning device becausethe reflective behaviour of the light reflecting aluminum layer dependson the pattern which the depressions form on the disc. Less light isreflected from a depression, frequently also called a groove, than froma raised area which is often also referred to as a land.

From the intensity of the light reflected from the CD disc, the opticalscanning device therefore recognizes whether the scanned bit relates forexample, to a logical one or a logical zero.

A further optical recording medium of this type, known under thedesignation of a magneto-optic disc, is described in the article"Magnetooptische Versuche dauern an" in Funkschau 13, 20th Jun. 1986 atpages 37-41.

In contrast to a conventional CD disc, a magneto-optic disc does nothave any pits. A magnetic layer, in which items of data are recordableand from which items of data are readable, is located behind thetransparent layer. It will now be explained how items of data arewritten onto a magneto-optic disc.

The magnetic layer is heated above the Curie temperature by means of alaser beam focused onto the disc. Usually however, it is only necessaryto heat up the magnetic layer to the compensation temperature which liessomewhat under the Curie temperature. An electromagnet, which magnetizesthe region heated by the laser beam in the one or the other direction ofmagnetization, is arranged behind the focal point on the disc. Because,after switching off the laser beam, the heated spot cools once morebelow the Curie temperature, the direction of magnetization determinedby the electromagnet is maintained: it is, so to speak, frozen in. Theindividual bits are stored in this manner in domains of differentdirections of magnetization. Thereby, the one direction of magnetizationof a domain corresponds, for example, to a logical one, while theopposite direction of magnetization represents a logical zero.

One makes use of the Kerr effect for reading the items of data. Theplane of polarization of a linearly polarized light beam is rotated bythe reflection at a magnetized mirror by a measurable angle. Independence upon the direction in which the mirror is magnetized, theplane of polarization of the reflected light beam is rotated to theright or to the left. However, because the individual domains on thedisc act like magnetized mirrors, the plane of polarization of ascanning light beam is rotated by a measurable angle to the left or tothe right in dependence upon the direction of magnetization of thecurrently scanned domain.

The optical scanning device recognizes which bit is present, a logicalone or a logical zero, from the rotation of the plane of polarization ofthe light beam reflected from the disc. In contrast to a CD disc havingpits, a magneto-optic disc is erasable and re-writable virtually asoften as desired.

A disc shaped recording medium which represents a combination of anoptical and a magneto-optic disc is known from DE-OS 37 32 875. Items ofdata are stored on this recording medium by means of pits and also inthe magnetic layer of the disc. Because the pits and the magneticdomains lie above one another, items of data are stored at one and thesame place in the form of pits as well as in the magnetic layer. Thestorage capacity of this disc is therefore twice as great as that of anormal optical disc or a magneto-optic disc.

An optical scanning device is discussed in the DE-OS 37 32 874 which issuitable for the three types of disc mentioned, since this opticalscanning device is able to read items of data from an optical disc, e.g.a compact disc, a magneto-optic disc as well as from a disc that isknown from the DE-OS 37 32 875.

In this optical scanning device, the light from a laser is focused ontothe disc and reflected from there to a polarization beam splitter which,in dependence on its direction of polarization, reflects it either ontoa first or a second photodetector. The data signal, which is stored inthe magnetic domains of the disc, is obtained from the difference of thephoto voltages of the first and the second photodetector. That datasignal, which reproduces the items of data stored on the disc by meansof the pits, is produced from the sum of the photo voltages of the firstand the second photodetector. The optical scanning device described inDE-OS 37 32 874 may, in a disc such as is specified in DE-OS 37 32 875,simultaneously read both the items of data stored by means of the pitsand the items of data stored in the magnetic domains.

However, because the pits likewise cause a--if only very small--rotation of the direction of polarization of the light emitted by thelaser, cross-talk between the data signal obtained by scanning the pitsand the data signal read from the magnetic domains with the aid of theKerr effect cannot be completely avoided.

The object of the invention therefore is to suppress the undesiredcross-talk as completely as possible using simple means.

SUMMARY OF THE INVENTION

The invention achieves this object in that, the recording mediumreflects the light beam through a second λ/2 plate and a λ/4 plate to asecond polarization beam splitter which guides the light beam independence on its direction of polarization onto a third or fourthphotodetector, that the output of the third and that of the fourthphotodetector are connected to the inputs of a second differentialamplifier, that the output of the first differential amplifier isconnected to the non-inverting input of a third differential amplifierfrom whose output the data signal which was obtained from the magneticlayer of the recording medium is derivable and that the output of thesecond differential amplifier is connected to the inverting input of thethird differential amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

There is shown in

FIG. 1 an embodiment of the invention;

FIG. 2 a cross-section through a magneto-optic disc having pits;

FIG. 3 the magnet-optic data signal which is subject to cross-talk;

FIG. 4 the interfering cross-talk signal;

FIG. 5 the magneto-optic data signal which is free of cross-talk.

DETAILED DESCRIPTION

The construction of the embodiment shown in FIG. 1 will now bedescribed.

The light produced by a light source, for example, a laser LS, shinesthrough a collimating lens KL, a prismatic beam splitter PS1 and anobjective lens OL onto a magneto-optic recording medium CD on which theitems of data are stored on top of each other in a magnetic layer and bymeans of pits. The light beam is focused onto the disc-likemagneto-optic recording medium CD, which will be subsequently referredto as magneto-optic discs, by means of the objective lens OL.

The magneto-optic disc CD reflects the light beam back to the objectivelens OL and to the prismatic beam splitter PS1 which deflects it througha right angle to a prismatic beam splitter PS3. The prismatic beamsplitter PS3 deflects one light beam through a right angle which thenpasses through a lens L5 and a cylindrical lens ZL onto a four quadrantphotodetector PD5 which is formed by four photo diodes A, B, C and D. Alight beam passes on in a straight line through the prismatic beamsplitter PS3 to a prismatic beam splitter PS2 and then further through aλ/2 plate and a λ/4 plate to a polarization beam splitter PO2. A lightbeam is deflected through a right angle by the prismatic beam splitterPS2 to a λ/2 plate from which the light beam passes on further to apolarization beam splitter PO1. The polarization beam splitter PO1guides the light beam in dependence on its direction of polarizationeither through a lens L1 to a photodetector PD1 or through a lens L2 toa photodetector PD2. The polarization beam splitter PO2 directs thelight beam coming from the λ/4 plate in dependence on its direction ofpolarization either through a lens L3 to a photodetector PD3 or througha lens L4 to a photodetector PD4.

The outputs of the two diagonally opposite photodiodes A and C of thefour quadrant photodetector PD5 are connected to a respective summinginput of a differential amplifier DV4, while the other two diagonallyopposite photodiodes B and D are connected to the subtracting inputs ofthe differential amplifier DV4. The two adjacent photodiodes A and D ofthe four quadrant photodetector PD5 are connected to the two summinginputs of a fifth differential amplifier DVS; the other two adjacentphotodiodes C and B of the four quadrant photodetector PD5 are connectedto the subtracting inputs of the differential amplifier DV5.

The output of the first photodetector PD1 is connected to the invertinginput of a differential amplifier DV1 and to the first input of asumming amplifier AV. The output of the photodetector PD2 is connectedto the non-inverting input of the differential amplifier DV1 and to thesecond input of the summing amplifier AV.

The output of the photodetector PD3 is connected to the inverting inputof a differential amplifier DV2, whose non-inverting input is connectedto the output of the photodetector PD4. The output of the differentialamplifier DV2 is connected to the input of an amplifier V whose outputis connected to the inverting input of a differential amplifier DV3. Theoutput of the differential amplifier DV1 is connected to thenon-inverting input of the differential amplifier DV3.

The manner in which the cross-talk is created will be explained with theaid of the cross-section through a magneto-optic recording medium, whichis shown in FIG. 2, on which items of data are stored on top of eachother in a magnetic layer as well as by means of pits.

A substrate layer is arranged on the magneto-optic disc behind amagneto-optic layer. However, the magneto-optic layer is not plane as ina conventional magneto-optic disc, but rather, it contains depressions,so-called pits, which serve for the storage of data as in an opticalcompact disc. If now, the linearly polarized light emitted by the laseris reflected at the magneto-optic layer MO of the disc, its direction ofpolarization will be rotated through a small angle either to the rightor to the left in dependence on the direction of magnetization of themagneto-optic layer as a result of the Kerr effect. However,elliptically polarized light is produced upon reflection at the edges ofthe pits, which is what causes the cross-talk in the magneto-optic datasignal.

As explained above, the light beam reflected from the disc is directedby the polarization beam splitter PO1 either onto the photodetector PD1or onto the photodetector PD2 in dependence on its direction ofpolarization. The magneto-optic data signal MSN, which reproduces theitems of data stored in the magnetic layer MO of the CD disc, is thusgenerated by forming the difference between the output signals of thetwo photodetectors PD1 and PD2 in the differential amplifier DV1.

The data signal PS, which reproduces the items of data stored by meansof the pits in the disc, is obtained by addition of the output signalsof the two photodetectors PD1 and PD2.

The magneto-optic data signal MSN present at the output of thedifferential amplifier DV1 which is shown in FIG. 3 is, however, stillsubject to the interfering cross-talk.

Now the invention is based on the perception that a λ/4 plate convertslinearly polarized light into elliptically polarized light. The lightreflected by the compact disc, which on the one hand is linearlypolarized in a particular plane in dependence on the direction ofmagnetization of the magnetic layer, and which on the other hand alsocontains elliptically polarized components as a result of the reflectionat the edges of the pits, shines on the prismatic beam splitter PS2which guides it not only to the polarization beam splitter PO1 but alsovia the λ/2 plate PL2 and the λ/4 plate PL3 to the polarization beamsplitter PO2. The λ/4 plate PL3 converts the linearly polarizedcomponent of the light, which represents the magneto-optic data signal,into elliptically polarized light. The elliptically polarized componentof the light, which is caused by the reflection at the edges of thepits, is, in contrast, converted into virtually linearly polarizedlight. The polarization beam splitter PO2 guides the light either ontothe photodetector PD3 or PD4 in dependence on its direction ofpolarization.

The signal ES, which is produced by the difference between the outputsignals of the two photodetectors PD3 and PD4 by means of thedifferential amplifier DV2, is shown in FIG. 4. Because themagneto-optic data signal is suppressed to a large extent in the signalES, the signal ES is the interference signal which is causing thecross-talk.

Consequently, by subtraction of the signal ES from the signal MSN in thedifferential amplifier DV3, a magneto-optic data signal MS, which iscompletely free of cross-talk and which reproduces the items of datastored in the magnetic layer MO of the CD disc, is produced at theoutput of the differential amplifier DV3. The magneto-optic data signalMS, which is free of cross-talk, is depicted in FIG. 5.

The amplification of the amplifier V, which serves for matching thecircuit arrangement consisting of the differential amplifiers DV1, DV2and DV3, can be determined experimentally for example. The amplificationof the amplifier V has to be altered until such time as the signal MS atthe output of the differential amplifier DV3 exhibits rectangular pulseshaving steep edges.

The cross-talk of a disc is dependent upon how great the component ofthe elliptically polarized light is in the light reflected from thedisc. The greater the component of the elliptically polarized light, thegreater the cross-talk. However, because the component of theelliptically polarized light varies from disc to disc in dependence onthe quality of the disc, the discs exhibit differing degrees ofcross-talk.

Consequently, one embodiment of the invention provides for measuring theslope of the edges of the signal MS at the output of the differentialamplifier DV3 and regulating the amplification of the amplifier V suchthat the slope of the edges is a maximum when reading a disc. Due tothis measure, the amplification of the amplifier V is optimally matchedto the specific reflective properties of the disc which is currentlybeing scanned. Consequently, the cross-talk is optimally eliminated foreach disc in dependence on the reflective properties of the discs.

The invention is suitable for an optical recording and/or reproductiondevice which can read both optical and magneto-optic recording media aswell as a combination of both. The invention can be advantageously usedfor data processing because items of data can be simultaneously read andrecorded. However, the invention also offers the advantage for example,that sound and pictures can be recorded in CD players and video discplayers simultaneously with the reproduction.

I claim:
 1. Optical scanning device which is suitable for reading bothan optical and a magneto-optic recording medium as well as amagneto-optic recording medium (CD) upon which items of data are storedon top of one another in both a magnetic layer (MO) and by means of pits(P), wherein the light beam from a light source (LS) is focused on therecording medium (CD) and is reflected by the recording medium (CD)through a first λ/2 plate (PL1) to a first polarization beam splitter(PO1) which guides the light beam in dependence on its direction ofpolarization onto a first or second photodetector (PD1, PD2), whereinthe output of the first and of the second photodetector (PD1, PD2) areconnected to respective inputs of a first differential amplifier (DV1),said scanning device further comprising:a second (DV2) and a third (DV3)differential amplifier having respective inverting and non-invertinginput connections and respective output connections, the outputconnection of the first (DV1) and second (DV2) differential amplifiersbeing respectively coupled to the non-inverting and inverting inputconnections of the third (DV3) differential amplifier from whose outputconnection a substantially crosstalk free data signal (MS) obtained fromthe magnetic layer MO of the recording medium is available; third (PD3)and fourth (PD4) photodetectors respectively connected to the invertingand non-inverting input connections of the second (DV2) differentialamplifier; a second polarization beam splitter (PO2) arranged to directlight reflected from the recording medium, in dependence on itsdirection of polarization, onto said third (PD3) and fourth (PD4)photodetectors; and a second λ/2 plate (PL2) and a λ/4 plate (PL3)arranged in an optical path of said reflected light between saidrecording medium and said second polarization beam splitter (PO2). 2.The optical scanning device set forth in claim 1, further comprising anequalizing amplifier (V) coupled between the second differentialamplifier (DV2) and the third differential amplifier (DV3).
 3. Theoptical scanning device set forth in claim 2, wherein the amplifier (V)has a gain function set such that the slope of the transitions in thesignal (MS) at the output of the third differential amplifier (DV3)attains a predetermined threshold value or is a maximum.
 4. The opticalscanning device set forth in claim 3, wherein the gain function of theamplifier (V) is automatically altered until such time as the slope ofthe transitions in the signal (MS) at the output of the thirddifferential amplifier (DV3) attains the predetermined threshold valueor is a maximum.
 5. The optical scanning device set forth in claim 2,wherein the equalizing amplifier (V) has a gain set such that the slopeof the transitions in the signal (MS) at the output of the thirddifferential amplifier (DV3) attains a predetermined threshold value oris a maximum.
 6. The optical scanning device set forth in claim 1further comprising:a collimating lens (KL), a first prismatic beamsplitter (PS1), and an objective lens (OL) arranged to focus a lightbeam from said light source onto the recording medium (CD), and todirect light reflected from said magneto-optic recording medium (CD) toa second prismatic beam splitter (PS2), said second prismatic beamsplitter (PS2) being arranged to direct said reflected light onto thefirst photodetector (PD1) via an optical path including the first λ/2plate (PL1), the first polarization beam splitter (PO1), and a firstlens (L1) and also to direct said reflected light onto the fourthphotodetector (PD4) via an optical path including the second λ/2 plate(PL2), the λ/4 plate (PL3), the second polarization beam splitter (P02),and a fourth lens (L4); a second lens (L2) in an optical path betweensaid first polarization beam splitter (PO1) and said secondphotodetector (PD2) for focusing said reflected light on said secondphotodetector; a third lens (L3) in an optical path between said secondpolarization beam splitter (PO2) and said third photodetector (PD3) forfocusing said reflected light on said third photodetector; respectivemeans for electrically coupling the first (PD1) and second (PD2)photodetectors to the inverting and non-inverting input connectionsrespectively, of the first differential amplifier; respective means forelectrically coupling the third (PD3) and fourth (PD4) photodetectors tothe inverting and non-inverting input connections respectively, of thesecond differential amplifier; and an amplifier (V), for coupling theoutput of the second differential amplifier (DV2) to the inverting inputof the third differential amplifier (DV3).
 7. The optical scanningdevice set forth in claim 6 further comprising:a fifth lens (L5) and acylindrical lens (ZL); a four-quadrant photodetector (PD5); a thirdprismatic beam splitter (PS3), in an optical path between the firstprismatic beam splitter (PS1) and the second prismatic beam splitter(PS2), and arranged to direct a light beam via said fifth lens (L5) andsaid cylindrical lens (ZL) onto said four-quadrant photodetector (PD5).8. The optical scanning device set forth in claim 7, wherein respectiveoutput connections of two diagonally opposite photodiodes (A and C or Band D) of said four-quadrant photodetector are respectively connected totwo summing inputs or subtracting inputs respectively of a fourthdifferential amplifier (DV4) from whose output a focusing error signal(FE) for a focusing control loop is derivable.
 9. The optical scanningdevice set forth in claim 8, further comprising:a fifth differentialamplifier (DV5); respective means for coupling output connections of afirst pair of diagonally opposite photodiodes of said four quadrantphotodetector to a first pair of inverting and non-inverting inputconnections of said fifth differential amplifier (DV5); respective meansfor coupling output connections of a second pair of diagonally oppositephotodiodes of said four quadrant photodetector to a second pair ofinverting and non-inverting input connections of said fifth differentialamplifier (DV5).
 10. The optical scanning device set forth in claim 8,wherein respective output connections of the first photodetector (PD1)and the second photodetector (PD2) are connected to respective inputconnections of a summing amplifier (AV) from whose output a data signal(PS) obtained from the pits of the recording medium (CD) is derivable.11. The optical scanning device set forth in claim 7, furthercomprising:a fifth differential amplifier (DV5); respective means forcoupling output connections of a first pair of diagonally oppositephotodiodes of said four quadrant photodetector to a first pair ofinverting and non-inverting input connections of said fifth differentialamplifier (DV5); respective means for coupling output connections of asecond pair of diagonally opposite photodiodes of said four quadrantphotodetector to a second pair of inverting and non-inverting inputconnections of said fifth differential amplifier (DV5).
 12. The opticalscanning device set forth in claim 11, wherein respective outputconnections of the first photodetector (PD1) and the secondphotodetector (PD2) are connected to respective input connections of asumming amplifier (AV) from whose output a data signal (PS) obtainedfrom the pits of the recording medium (CD) is derivable.
 13. The opticalscanning device set forth in claim 6, wherein respective outputconnections of the first photodetector (PD1) and the secondphotodetector (PD2) are connected to respective input connections of asumming amplifier (AV) from whose output a data signal (PS) obtainedfrom the pits of the recording medium (CD) is derivable.
 14. The opticalscanning device set forth in claim 6, wherein the amplifier (V) has again function set such that the slope of the transitions in the signal(MS) at the output of the third differential amplifier (DV3) attains apredetermined threshold value or is a maximum.
 15. The optical scanningdevice set forth in claim 7, wherein respective output connections ofthe first photodetector (PD1) and the second photodetector (PD2) areconnected to respective input connections of a summing amplifier (AV)from whose output a data signal (PS) obtained from the pits of therecording medium (CD) is derivable.
 16. The optical scanning device setforth in claim 7, wherein the amplifier (V) has a gain function set suchthat the slope of the transitions in the signal (MS) at the output ofthe third differential amplifier (DV3) attains a predetermined thresholdvalue or is a maximum.
 17. The optical scanning device set forth inclaim 1, wherein respective output connections of the firstphotodetector (PD1) and the second photodetector (PD2) are connected torespective input connections of a summing amplifier (AV) from whoseoutput a data signal (PS) obtained from the pits of the recording medium(CD) is derivable.
 18. The optical scanning device set forth in claim 1further comprising:a further lens (L5) and a cylindrical lens (ZL); afour-quadrant photodetector (PD5); a prismatic beam splitter (PS3), inan optical path of reflected said light beam, and arranged to direct aportion of said light beam via said further lens (L5) and saidcylindrical lens (ZL) onto said four-quadrant photodetector (PD5). 19.The optical scanning device set forth in claim 18, wherein respectiveoutput connections of two diagonally opposite photodiodes (A and C or Band D) of said four-quadrant photodetector are respectively connected totwo summing inputs or subtracting inputs respectively of a fourthdifferential amplifier (DV4) from whose output a focusing error signal(FE) for a focusing control loop is derivable.